Fluid catalytic cracking (FCC) is an important process for the petroleum refinement. In china, about 70-80% of gasoline is obtained from the FCC process. Due to the increased environmental awareness and the continuing issuance of the stricter environment protection regulations and the stricter emission standard, the requirement on the quality of FCC gasoline is being increased, especially for the olefin content. Therefore there is a need in the art for a catalytic cracking catalyst having a capability of reducing the olefin content.
The currently used active component for reducing the olefin content in gasoline is mostly the rare earth-containing Y zeolite. References can be made to CN1317547A, CN1506161A, CN101537366A, CN1436727A, CN1382631A, CN101823726A, CN100344374C, CN1053808A, CN1069553C, CN1026225C, and CN101147875A.
For example, CN101147875A discloses a catalytic cracking catalyst, which contains a high rare earth ultra stable Y zeolite. Said high rare earth ultra stable Y zeolite is prepared as follows. An ultra stable Y zeolite used as starting material is mixed with an acid. The mixture is stirred, washed and filtered. To the mixture is added a solution of rare earth salt to conduct an exchange. Then the mixture is washed, filtered and dried. The product of the ratio of the strength I1 of the peak at 2θ=11.8±0.1° to the strength I2 of the peak at 2θ=12.3±0.1° (I1/I2) in the X-ray diffraction spectrogram of said high rare earth ultra stable Y zeolite and the rare earth content in the zeolite (RE2O3%) is not higher than 40.
The rare earth-containing Y zeolite can be prepared through a one-exchange-and-one-calcination procedure (the ion exchange is conducted once and the high temperature calcination is conducted once, references can be made to, for example, CN1436727A, CN101823726A and CN100344374C), or a two-exchange-and-two-calcination procedure (the liquid-phase rare earth ion exchanges are conducted twice and the high temperature calcinations are conducted twice, references can be made to, for example, CN1506161A and CN101537366A).
For either the prior two-exchange-and-two-calcination procedure or the prior one-exchange-and-one-calcination procedure, in the preparation of the rare earth-containing Y zeolite, the amount of rare earth in the product is usually lower than the total fed amount of rare earth. Even if many rare earth ions are located in sodalite cages, it is inevitable that some rare earth ions are still present in super cages. The rare earth ions in super cages can be backwashed off in the subsequent washing procedures, resulting in the loss of rare earth and the decrease of the rare earth utilization.
In the current industry, for saving the production cost and increasing the product efficiencies, upon the preparation of the Y zeolite having high rare earth content, a two-exchange-and-one-calcination procedure is mostly used. That is to say, after the first calcination, the rare earth exchange is conducted once more, but the second calcination is not done. Due to the lack of the second calcination, the effective migration of rare earth ions cannot be achieved, and most of rare earth ions are still present in super cages. The rare earth ions in super cages can be backwashed off in the subsequent washing procedures, also resulting in the decrease of the rare earth utilization.